There is a great deal of interest in developing better and more efficient means for storing energy for applications such as radio communication, satellites, portable computers, and electrical vehicles, to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective battery cells having improved performance characteristics. Electrochemical battery cells are preferred and hence widely used in these applications since the chemical reactions which take place in the cells can be converted into useful electrical energy.
An electrochemical battery cell uses its reactive components, namely the positive and negative electrodes, to generate an electric current. The electrodes are separated from one another by an electrolyte which maintains ionic conduction between the two electrodes. Electrons flow from one electrode through an external circuit to the other electrode completing the circuit. Rechargeable, or secondary, cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions are reversible. Accordingly, electrodes for secondary cells must be capable of being regenerated (i.e. recharged) many times. The development of advanced rechargeable cells depends on the design and selection of appropriate materials for the electrodes and the electrolyte.
Currently, electrolytes used in rechargeable electrochemical cells include a polymer, such as a polyethylene oxide (PEO), impregnated with a salt in a solvent. These salts include, for example, LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiCF.sub.3 SO.sub.3, LiN(CF.sub.3 SO.sub.2).sub.2, and LiClO.sub.4 to name but a few. The solvents include ethylene carbonate (EC), propylene carbonate (PC), glycol diethers (glymes), and combinations thereof.
A schematic representation of the prior art electrochemical cell is illustrated in FIG. 1. Specifically, FIG. 1 illustrates a positive electrode 10, and a negative electrode 20. Disposed between said positive and negative electrodes is the electrolyte 30. The positive electrode/electrolyte interface 32 is created at the boundary between the positive electrode 10 and the electrolyte 30. Similarly, a negative electrode/electrolyte interface 34 is created at the boundary of the negative electrode 20 and the electrolyte 30. As may be appreciated from a perusal of FIG. 1, the electrolyte is in direct contact with both the negative and positive electrodes.
Present day state-of-the-art polymer electrolyte batteries are often plagued by electrode/electrolyte incompatibilities. Specifically PEO, EC, PC, and glymes all include carbon/oxygen bonds which are particularly reactive with the lithium in lithium batteries. Thus, at the negative electrode/electrolyte interface 34, the negative electrode and the electrolyte solvent may react to form an ionically insulating layer on the surface of the negative electrode material. This ionically insulating layer blocks the transport of lithium ions, and reduces active material reaction sites, effectively isolating the negative electrode. The result is reduced capacity with cycling (i.e., repeated charge/discharge) and poor cycle life.
At the positive electrode/electrolyte interface 32, problems include thermodynamic instability of the electrolyte due to the higher electrode potentials of the positive electrode. This causes electrolyte decomposition, and electrolyte outgassing. The result of outgassing and decomposition is increased internal cell pressure, reduced cell performance, and ultimately, explosive cell failure. The problem of electrolyte decomposition has been addressed by operating the cell at less than the positive electrode's half cell potential. That is, at a potential at which the electrolyte remains stable. However, this results in decreased energy density, and a decreased charge per cycle.
Another problem inherent in current electrode/electrolyte pairs relates to disruptions in the contact between the electrolyte and the electrodes, resulting from thermal fluctuations during cycling. That is, as the electrochemical cell is cycled, the lithium anode swells and contracts. This results in loss of contact between the anode and the electrolyte, and accordingly, failure of the cell.
Accordingly, there exists a need for an electrolyte which has enhanced conductivity, yet reduces or eliminates the deleterious reactions characterizing prior art electrolytes.